To reduce dissipation losses in the output stage, most audio frequency amplifiers comprise a class B or AB amplifier. A conventional class B or AB amplifier comprises a pair of emitter-coupled output transistors configured in a push-pull arrangement. The audio frequency signal is applied to the bases of these output transistors and the amplifies audio frequency signal is present at the emitters thereof. In a conventional push-pull amplifier, the collector of one of the output transistors is connected to a fixed positive DC voltage to provide a current source and the collector of the other of the output transistors is connected to a fixed negative DC voltage to provide a current sink. The positive and negative DC voltages are commonly referred to as the source and sink voltages, respectively.
With a reactive load, the entire fixed voltage is present across the amplifying transistor of a class B or AB amplifier when current flows to the load, yielding high dissipation losses. These losses are particularly high with high dynamic range signals such as music signals. Accordingly, while more efficient than a pure class A amplifier, class B and AB amplifiers still exhibit significant dissipation losses with resistive loads and exhibit considerably greater losses with reactive loads. At the high power levels of modern audio amplifiers, these dissipation losses require the use of numerous expensive high power semiconductors in parallel and also that extensive steps be taken to cool the output transistors.
To obtain higher efficiencies, class D amplifiers have also been proposed as audio amplifiers. Class D amplifiers comprise a power transistor, a low pass filter, and a freewheeling diode or rectifier in parallel with the low pass filter. The power transistor is switched on and off according to a high frequency square wave signal the pulse width of which is modulated according to the audio frequency signal to be amplified. The filter then recovers the audio frequency signal by filtering off the high frequency square wave signal. The class D amplifier achieves high efficiencies by delivering current only when substantially zero volts are present across the power transistors.
While significantly more efficient than a class B or AB amplifier, a class D amplifier has high distortion that cannot be easily corrected with negative feedback because of the phase shift introduced by the low pass filter. Further, the low pass filter tends to interact with the load in an undesirable fashion. Also, because of the relatively high frequencies involved, class D amplifiers exhibit poor supply rejection and thus are highly susceptible to power supply disturbances. For these and other reasons, class D amplifiers have not been used in commercial audio amplifiers.
Two other classes of high efficiency audio frequency amplifiers have been proposed to increase the efficiency of output transistors arranged in a push-pull configuration. These amplifiers are referred to as class G and class BD amplifiers and employ adaptive power supplies for generating source and sink voltages for a push-pull amplifier. These adaptive power supplies generate source and sink voltages that increase and decrease as the audio frequency signal increases and decreases. The basic idea with these amplifiers is to provide high voltage to the push-pull amplifier only when the audio frequency signal is high. As an audio frequency signal developed from a musical source is normally relatively low with infrequent high bursts, class G and BD amplifiers used as audio amplifiers normally maintain the voltage across the power transistors at a low level, thereby greatly reducing the average power dissipation of the output transistors.
A class G amplifier normally comprises a push-pull amplifier and a stepped power supply that generates source and sink voltages that are increased and decreased in two or three discrete steps as the power requirement of the signal being amplified increases and decreases. Such an amplifier is disclosed, for example, in U.S. Pat. No. 4,484,150 to Carver and U.S. Pat. No. 3,961,280 to Sampei.
A class BD amplifier conventionally comprises a pair of highly efficient class D amplifiers to provide signal tacking source and sink voltages to a push-pull amplifier. Class BD amplifiers are generally discussed in the following articles: (a) The Class BD High-Efficiency RF Power Amplifier dated June 1977 and written by Frederick H. Raan; and (b) An Amplifier With A Tracking Power Supply dated Nov. 5, 1973, and written by V. M. Kibakin.
The present invention is particularly useful when implemented in the context of audio amplifiers containing tracking power supplies, and that application will be discussed detail herein. However, the present invention has broader application to buck converters in which the output is designed to track a fine-varying input signal. Accordingly, the scope of the present invention should be determined according to the claims appended hereto and not the following detailed discussion.
Tracking power supplies can be classified as envelope trackers, rail-to-ground tracker, and rail-to-rail trackers; the present invention may be used to advantage in each of these configurations.
Exemplary envelope trackers are disclose din U.S. Pat. No. 3,426,290 issued 4 Feb. 1969 to Jensen, U.S. Pat. No. 4,218,660 issued 19 Aug. 1980 to Carver, and, more recently, U.S. Pat. No. 5,075,634 issued 24 Dec. 1991 to French.
Rail-to-ground tracking power supplies are disclosed in U.S. Pat. No. 4,054,843 issued 18 Oct. 1977 to Hamada, U.S. Pat. No. 4,409,559 issued 11 Oct. 1983 to Amada, an article published by the Audio Engineering Society in 1981 entitled A HIGH EFFICIENCY AUDIO POWER AMPLIFIER (Nakagaki and Amada), and U.S. Pat. No. 4,507,619 issued 26 Mar. 1985 to Dijkstra.
Rail-to-rail tracking power supplies are disclosed in U.S. Pat. No. 4,087,759 issued 2 May 1978 to Iwamatsu, U.S. Pat. No. 4,472,687 issued 18 Sep. 1984 to Kashiwagi et al., and U.S. Pat. No. 5,200,711 issued 6 Apr. 1993 to Anderson et al.
In an amplifier having any one of these three types of tracking power supplies, the collector-emitter voltage across the amplifying transistor will ideally remain substantially constant at a low value. The basic advantage of all types of tracking power supplies is thus that dissipation losses in the amplifying transistor are reduced. Additionally, in tracking power supplies, dissipation losses of the amplifying device are ideally kept low for both resistive and reactive loads.
A tracking power supply in a class BD amplifier will thus in general reduce by varying degrees the dissipation of the output transistor relative to the dissipation of the output transistors in a pure class B or AB amplifier. Rail-to-rail tracking power supplies are theoretically the most efficient, while envelope tracking power supplies are the least efficient of the three types of tracking power supplies. Rail-to-ground and rail-to-rail tracking power supplies will also have the additional advantage of reducing the voltage rating requirements of the output transistors.
As mentioned, in theory the most efficient of the various amplifier configurations having tracking power supplies is the rail-to-rail tracking power supply as described above.
In related U.S. patent application Ser. No. 08/154,739 assigned to the Assignee of this application, the Applicants recognized that prior art amplifiers having signal tracking powers supplies do not precisely track the signal being amplified; instead, the source and sink supply voltages deviate from their theoretical levels under the following conditions: (a) high frequency audio signals; (b) open circuit or light loads; (c) certain reactive loads; (d) asymmetric signals; and/or (e) high offset voltages of a tracking power supply is referred to as floating.
The '739 application further recognized that this floating occurs because, under the conditions described above, insufficient current flows through the amplifying transistor to pull the supply voltage towards the reference when the audio frequency signal being tracked moves towards the reference. In particular, a class BD amplifier comprises a class B or AB output stage and a power supply containing source and sink output filters, each output filter comprising an inductor and an output capacitor. When little or no current is being drawn by the output stage, no current flows back through the inductors of the output filters to discharge the output capacitors. The source and sink voltages thus tend to hang or float until the output devices begin to draw current to discharge the output capacitors. The difference between the floating source supply voltage and the plunging sink supply voltage can become very large, and this large voltage difference can last from one cycle of the audio frequency signal to the next cycle thereof.
The large voltage across the output stage caused by floating can result in high dissipative losses in the output devices and thus requires high power transistors with a large safe operation area. The large voltages that can momentarily develop across the amplifier output stage also require that the transistors have a high breakdown voltage. Without high dissipative capacity and high breakdown voltage, the likelihood that the output devices will fail under the conditions during which floating occurs is greatly increased.
The '739 application thus proposed an audio frequency amplifier comprising a signal tracking power supply having at least one output filter and further comprising discharge means for discharging an output capacitor of the power supply output filter, thereby ensuring that the source supply voltage follows the audio frequency signal back down and/or that the sink supply voltage follows the audio frequency signal back up after the slope of the audio frequency signal changes signs.
Discharge means as described in the '739 application will guarantee that the source and sink supply voltages will not float. This results in a predetermined maximum voltage value across the output stage. Therefore, by setting this predetermined maximum voltage value at a low level, low voltage devices can be used in the output stage of a rail-to-rail tracking power supply and, to a lesser extent, of a rail-to-ground tracking power supply.